Orchestra des Champs Elysées/Herreweghe, Royal Festival Hall, LondonThe Cunning Little Vixen, Royal Opera House, London

Though Flemish conductor Philippe Herreweghe made his name with the choral music of Bach and Schütz, few would disagree that what suits him best is the repertoire he has been exploring with the Orchestre des Champs Elysées. In much the same manner as Sir Roger Norrington's pioneering work with the London Classical Players, Herreweghe and the OCE have peeled back preconceptions of 19th-century sonority; revealing the intensity of the late-Classical and early-Romantic eras' successive developments in harmony, orchestration and expressive intention through the concentrated sound of original instruments.

Though Flemish conductor Philippe Herreweghe made his name with the choral music of Bach and Schütz, few would disagree that what suits him best is the repertoire he has been exploring with the Orchestre des Champs Elysées. In much the same manner as Sir Roger Norrington's pioneering work with the London Classical Players, Herreweghe and the OCE have peeled back preconceptions of 19th-century sonority; revealing the intensity of the late-Classical and early-Romantic eras' successive developments in harmony, orchestration and expressive intention through the concentrated sound of original instruments.

Herreweghe's approach has transformed many listeners' attitudes to Berlioz, Mendelssohn and Fauré; rendering repertoire as familiar as Nuits d'Eté, A Midsummer Night's Dream and the Requiem newly earthy and immediate to experienced ears and demonstrating that the modern conceit of authenticity has a function beyond academic fancy. So are the days of his dessicated choral projects now over? On the basis of this week's Mozart programme with the OCE and his first choir Collegium Vocale Gent, it would seem they're not.

But first to the orchestra; full-bodied of sound, explosive in dynamic range, beautifully tuned, miraculously responsive, wholly engaged, and richly versatile in articulation. Did Herreweghe's psychiatric studies play a part in his reading of Mozart's Symphony No 39 (a work written at a time of acute personal crisis)? Almost certainly. Though the sun-baked optimism and verdant sweetness of this symphony were warmly layed out, though the minuet was as irresistibly entertaining as a small rubber ball, and the smoky blush of basset-horns as careless as a kiss, the yelps of distress and despair were as searing, sudden and ferrous-flavoured as paper-cuts on your tongue. But there's nothing that can underline dissonance as boldly as the high-risk, knife-twisting tonal clash between gut strings and natural horns.

If Symphony No 39 showed how dramatic and interesting a musician Herreweghe has become, the Mass in C minor only showed that as soon as he has Collegium Vocale Gent back in front of him and a series of quavers that can – at a severe stylistic stretch – be shaped into something almost baroque, musical recidivism rules the day. No matter that those who've stuck to earlier repertoire pull far richer sounds than these from their vocal ensembles – that colourless 1970s choral sound is still Herreweghe's ideal.

Of course it's the sopranos who get it in the neck here, and literally so. Plenty of conductors are happy for their basses to sing freely and naturally while expecting the sopranos to imitate little white mice – though the result of suppressing an adult voice in this fashion is a larynx so taut and high that you might as well wear it as an alice band. Consequently, both blend and intonation were poor on top, though Collegium Vocale Gent's altos, tenors and basses produced a lively sound and all voice parts shaped the desanguinated interjections of "Suscipe, suscipe" sensitively. But for a work that is as strongly rooted in the flash and fire of opera seria as it is in the Scarlattian vocabulary of double-dots and triple fugues, this was a curiously one-sided, one-dimensional account that saw even Herreweghe's soloists straitjacketed.

Another refugee from the early music annexe can be heard at Covent Garden this month, though conductor Sir John Eliot Gardiner has not taken to The Cunning Little Vixen like a duck to water. Ensemble problems which, to be fair, are seemingly intrinsic to this score were rife in his first performance, while the lack of anything approaching luminosity, or blood, or mud, or heartbeat to bind Janacek's fleeting orchestral colours made heavy weather of this brutal yet cloyingly sentimental opera.

Though William Dudley's clever sets – a busy cocktail of commercial, mechanical and natural motifs from the 1920s – still produce genuine gasps of pleasure, Bill Bryden's fussy, cutesy, over-populated production tries far too hard to be likeable. (Dancer Tom Sapsford's Blue Dragonfly in particular made me long for a very large fly swatter.) But this is Dawn Upshaw's show and she sings the role of Sharp-Ears sweetly and plainly, milking her naturally sympathetic sound and direct diction for all their worth. Though Upshaw's stage-manner is noticeably more self-conscious than those of Joyce DiDonato (Fox), Jonathan Veira (Harasta), old hands Stuart Kale (Schoolmaster) and Elizabeth Sikora (Innkeeper's wife), and the up-and-coming duo of Rachel Nicholls and Sarah Fox (Pepik and Frantik), and though no one, but no one, can outsing or outact the ever wonderful Gerald Finley (Gamekeeper), hers is a most welcome and long overdue house debut.

a.picard@independent.co.uk

'The Cunning Little Vixen': Royal Opera House, London WC2 (020 7304 4000) to Weds